1.0 Introduction

"Normals" is the term commonly used for values of climatic elements
averaged over a fixed, standard period of years. In the 1930's, the
30-year period from 1901 to 1930 was selected as the first
international standard period. The selection of this number of years
was somewhat arbitrary, however it was considered sufficiently long to
eliminate year-to-year variations. In 1960 it was agreed that
countries should continue to use 30-year periods, but that normals
should be updated every decade rather than every 30 years, beginning
with the period 1931 to 1960.

In Canada in the first part of this century there were a limited
number of stations with observations over the 1901-1930 period. As a
result, averages were computed based on all available data, rather
than the normals period. The first set of 30-year normals was computed
for the 1921-1950 period, using adding machines. Sets were produced
after the end of each subsequent decade. The first computer for
climate applications was installed in 1965, in part to facilitate
normals production.

2.0 Climate Elements

2.0 Climate Elements

Except where labelled "extreme", values of the climate elements are
averages for the 1961-90 period, or for a portion of that period no
shorter than 20 years. Extreme values are the highest or lowest
occurrence for all years for which data were available. Extremes whose
corresponding means are missing should be used with caution. They are
often derived from less than twenty years of observations, and may not
be indicative of occurrences which could be expected over longer
periods.

The starting and ending dates for station observing programs are for
the total period of observation, and hence refer to the period used to
calculate the extreme values.

Symbols used with the data include "+" to indicate a value occurred
more than once in a given period and "M" to indicate there are no data
for the period. "N" or "X" indicates that some data do exist, but not
enough to derive a value. See "Missing Data" below for more
information. In cases where a "+" appears, dates given are for the
most recent event. Where a "*" replaces a number in the "days with"
section, the quantity is less than one.

In the following discussion, stations are categorized as either
"ordinary" or "principal". The former are typically stations that
record daily temperature and/or precipitation amounts. In contrast
principal stations take hourly observations for all or parts of the
day. In the following tables, stations for which elements such as
pressure, relative humidity and wind are included are usually
principal.

2.1 Temperature

2.1 Temperature

Temperatures are measured in a louvered box called a Stevenson
screen, mounted 1.5 m above the ground, which is usually a level,
grassy surface. At most ordinary stations the maximum temperature is
the highest recorded in a 24-hour period ending in the morning of the
next day. The minimum values are for a period of the same length,
beginning in the evening of the previous day. Mean temperature is the
average of the two. At most principal stations the maximum and minimum
temperatures are for a day beginning at 0600 Greenwich (or Universal)
Mean Time, which is within a few hours of midnight local standard time
in Canada.

Dry-bulb temperature, like daily maximum and minimum temperatures,
is measured in the Stevenson screen, which protects the thermometers
from direct sunlight. Dry-bulb temperatures are recorded hourly at
principal climate stations. Values given are hourly averages for each
month and year, for all hours combined, and for each individual hour.

2.2 Degree-Days

2.2 Degree-Days

Degree-days for a given day represent the number of degrees Celsius
that the mean temperature is above or below a given base. For example,
heating degree-days are the number of degrees below 18°C. If the
temperature is equal to or greater than 18°C, then the number will
be zero. Values above or below the base of 18°C are used primarily
to estimate the heating and cooling requirements of buildings. Values
above 5°C are frequently called growing degree-days, and are used
in agriculture as an index of crop growth. Values in the tables
represent the average accumulation of degree-days, above or below a
selection of base temperatures, for a given month or year.

2.3 Season of Minimum Temperature Above 0°C

2.3 Season of Minimum Temperature Above 0°C

Winds are normally measured at level, open sites removed as much as
possible from obstacles to wind flow such as trees, buildings, or
hills.

2.4 Precipitation

2.4 Precipitation

Rain, drizzle, freezing rain, freezing drizzle and hail are usually
measured using the standard Canadian rain gauge, a cylindrical
container 40 cm high and 11.3 cm in diameter. The precipitation is
funnelled into a plastic graduate which serves as the measuring
device. Snowfall is the measured depth of newly fallen snow, measured
using a snow ruler. Measurements are made at several points which
appear representative of the immediate area, and then averaged.
"Precipitation" in the tables is the water equivalent of all types of
precipitation.

At most ordinary stations the water equivalent of snowfall is
computed by dividing the measured amount by ten. At principal stations
it is usually determined by melting the snow that falls into Nipher
gauges. These are precipitation gauges designed to minimize turbulence
around the orifice, and to be high enough above the ground to prevent
most blowing snow from entering. The amount of snow as determined by
this method normally provides a more accurate estimate of
precipitation than using the "ten-to-one" rule. Even at ordinary
stations the normals precipitation values will not always be equal to
rainfall plus one tenth snowfall. Missing observations is one cause of
such discrepancies.

Precipitation measurements are usually made four times daily at
principal stations. At ordinary sites they are usually made once or
twice per day. Rainfall, snowfall and precipitation amounts given
represent the average accumulation for a given month or year.

2.5 Snow Cover

2.5 Snow Cover

Snow cover is the depth of accumulated snow on the ground, measured
at several points, which appear representative of the immediate area,
and then averaged. End-of-month values are given.

2.6 Number of Days With Specified Parameters

2.6 Number of Days With Specified Parameters

These give the average number of days per month or year on which a
specific meteorological event occurs.

In the case of rainfall and precipitation, 0.2 mm or more must
occur before a "day with" is counted. The corresponding figure for
snowfall is 0.2 cm.

A day with freezing precipitation is counted if there is an
occurrence of 0.2 mm or more of rain or drizzle, which turns to ice
on contact with the underlying surface.

Fog for this purpose is defined as a suspension of very small
water droplets reducing the horizontal visibility to less than 1 km.

A day with thunderstorms occurs if thunder is heard.

Days with smoke/haze, blowing dust or blowing snow are counted if
the horizontal visibility is reduced to 10 km or less by these
elements.

A day with hail is counted when hail, ice pieces 5 mm or more in
diameter, occur at the observing site.

2.7 Pressure

2.7 Pressure

Station pressure is the force exerted on the earth's surface by a
column of air of unit cross-sectional area extending from the surface
of the earth to the outer limit of the atmosphere. It is given in
kilopascals: one kilopascal is equal to 10 millibars, 0.2953 inches of
mercury, or 0.145 pounds per square inch. The standard instrument for
the measurement of atmospheric pressure is the mercury barometer, in
which the air pressure is balanced against the weight of a column of
mercury in a glass tube that contains a vacuum.

Sea level pressure is computed by adding to the observed station
pressure the equivalent weight of an air column extending from sea
level to the station elevation.

2.8 Moisture

Vapour pressure is the pressure exerted by the moisture in the air.
It increases with both atmospheric temperature and moisture content.
It is related to relative humidity in that the latter is the ratio of
the actual value of vapour pressure to its value if the air were
saturated at the same temperature. For a note on the unit of
measurement, see the section on station pressure.

Relative humidity and vapour pressure are derived from standard air
temperature measurements, and directly measured moisture parameters,
such as wet-bulb temperature. Dew-point temperature, similarly
derived, is also an indicator of moisture in the air.

2.9 Wind

2.9 Wind

The majority of wind measurements are made by anemometers installed
at ten metres above the ground. A substantial minority of sites have
instruments installed at other heights, usually greater than ten
metres. Wind in the first ten's of metres above the ground tends to
increase in speed and veer with height.

Winds are normally measured at level, open sites removed as much as
possible from obstacles to wind flow such as trees, buildings, or
hills. At the majority of principal stations, wind is measured by
taking a one- or (since 1985) two-minute mean at each observation,
from a U2A anemometer. At other wind-measuring sites, values are
usually obtained from autographic records of U2A or 45B anemometers.
Averaging periods may vary from one minute to an hour. Winds measured
by U2A's are recorded to the nearest ten degrees, while those from the
45B provide them to eight points of the compass. The extreme gust
speed is the instantaneous peak wind observed from the anemometer
dials, or abstracted from a continuous chart recording.

Where directions were measured more precisely than eight points,
they have been converted to this format. The direction is defined as
that from which the wind blows.

2.10 Sunshine and Solar Radiation

2.10 Sunshine and Solar Radiation

In Canada, bright sunshine observations are made using the
Campbell-Stokes sunshine recorder, first developed in 1863. It
consists of a 10-cm glass sphere which focuses sunlight on a card
calibrated in hours. Sunlight burns a trace on the card, allowing the
observer to determine to the nearest tenth of an hour the amount of
sunshine that occurs on a given day. It should be noted that the
recorder measures only "bright" sunshine, which is less than "visible"
sunshine. For example, sunshine immediately after sunrise and just
before sunset would not be bright enough to register. Values given are
the totals for each month and year.

Solar radiation, which is a measure of the sun's electro-magnetic
energy, is available for a small number of stations across Canada.
These data are presented for each measured radiation field, namely,
global (RF1), diffuse (RF2), reflected (RF3), and net (RF4) radiation.

2.11 Visibility and Cloud Cover

2.11 Visibility and Cloud Cover

The number of hours (or observations) in three classes, of
visibility in kilometres (< 1 km, 1-9 km, > 9 km), and cloud
cover in tenths of sky covered (0-2/10, 3-7/10, 8-10/10), are computed
for principal stations.

2.12 Soil Temperature

2.12 Soil Temperature

Monthly averages of soil temperatures measured at a selection of
standard depths (5, 10, 20, 50, 100, 150 and 300 cm) are given.
Morning and afternoon averages are also given for depths of 5, 10 and
20 cm.

2.13 Evaporation

2.13 Evaporation

Monthly averages of calculated lake evaporation (mm) derived from
evaporation pan measurements are given. Lake evaporation represents
water loss from ponds and small reservoirs but not large lakes. Lake
evaporation tends in general to be about a third less than measured
pan evaporation.

3. Accuracy and Representativeness of Data

3. Accuracy and Representativeness of Data

The normals values are derived from data in the national
climatological archive of Environment Canada. While considerable
effort is made to ensure the accuracy of these data, no guarantee can
be given that they are error free.

The question of the extent to which climate elements measured at one
site are representative of surrounding locations is often raised.
There is no simple answer to this question, with factors such as the
magnitude of the distance from the observing site, homogeneity of the
terrain, and nature of the element having to be considered. For
example, over rough ground or in a built-up area, wind speeds can vary
dramatically over distances of a few metres. In contrast, air
temperature tends to be a less variable weather element. Over flat,
uniform terrain temperature measurements at a single site may be
representative of conditions tens of kilometres, or even further,
away. Expert advice should be sought when using these data at
locations where representativeness is in question.

4. Missing Data

4. Missing Data

Except for extremes, the following rules were used to determine
whether monthly values would be calculated. In the case of daily
temperatures (maximum, minimum and mean values) the "three-five rule"
was used, i.e., a month was counted as missing if more than five daily
observations were missing (more than three, if consecutive).

In the case of average monthly amounts of rain, snow and
precipitation, months were excluded if one or more days were missing.
The same method was used for elements which are accumulative, such as
degree-days and "days with". In the case of extreme values, no data
were excluded from the selection process. The "three-five" rule was
used for most of the remaining elements.